(a) Field of the Invention
The invention relates to improvements in electromagnetic wave receiver systems having AFC/AFT circuits and adjustable tuning means. More specifically, the invention relates to a circuit for automatically disabling the AFC/AFT when the system is being tuned.
(b) Description of Prior Art
It is known to use Automatic Frequency Control (AFC) and Automatic Fine Tuning (AFT) circuits for maintaining the tuning of frequency of receivers such as FM receivers and television receivers lined to the frequency of a selected radio or television station. Although AFC/AFT is useful for maintaining the alignment of frequencies when the receiver has been tuned, the signals provided by the AFC/AFT could provide problems when attempting to tune to a different station or channel. Specifically, if the AFC/AFT signal is strong enough, then the AFC will "pull" the receiver to the frequency at which it was last set even though the tuning dial is being turned. The AFC/AFT will only "let go" after the distance between the originally set frequency and the tuning dial is sufficiently large to overcome the effect of the AFC/AFT signal.
If there are any stations transmitting at frequencies in the space between the frequency at which the receiver was originally set and the frequency at which the AFC/AFT lets go, the output will not be detected by the receiver because of this effect on the AFC/AFT signal. For this reason, especially in FM receiver sets, there is provided an AFC ON/OFF switch whereby the AFC can be manually disabled during tuning. However, the purpose of this switch is usually not understood by the user of the set, so that the AFC/AFT is very seldom manually disabled when the set is being tuned. Even if it were understood, as it is left to the user to manually disable the AFC/AFT, it is quite conceivable that the switch will never be used. If it is used, it is possible that the user will forget to enable the AFC/AFT once he has completed tuning, so that the receiver will be operating without benefit of the AFC/AFT circuit.
(c) Objects
It is therefore an object of the invention to provide an improvement to electromagnetic wave receiver systems as above-described wherein the AFC/AFT is automatically disabled on tuning.
It is a more specific object of the invention to provide a circuit which senses when tuning is taking place and which automatically disables the AFC/AFT at such time.